Audio standards, such as MPEG-1, Layer 3 (also known as, and now referred to as MP3) employ lossy and lossless compression to reduce the memory and bandwidth requirements for storing and transmitting audio data.
During lossy compression, some of the original data is lost. Lossy compression includes digitization, windowing, time to frequency domain transformation, and quantization. A stochastic model of the human ear determines imperceptible portions of the original data. Accordingly, lossy compression realizes significant compression without perceptible degradation of the original signal. After lossy compression, the audio signal is represented by a series of symbols.
Lossless compression uses a variety of variable length codes for coding the symbols. The variable length codes for the symbols are designed to assign shorter codes to the most frequently occurring symbols and longer codes to the least frequently occurring symbols. The coding schemes include a number of tables that map the different symbols to different codes.
The encoded audio signal can then be transmitted and stored at a receiving terminal with an audio decoder. During play of the audio signal, the audio decoder decodes the variable length codes, inverse quantizes, transforms to the time domain, and dewindows the encoded audio signal, thereby reconstructing the original audio signal. Preferably, the foregoing occurs in real time, because most applications would require playing the audio signal at a specified speed.
The audio decoder is usually an integrated circuit. The audio decoder uses tables that map the different symbols to different codes to decode the variable length codes. The tables occupy approximately 50 KB of memory. In an integrated circuit, the amount of on-chip memory is limited and expensive. Although off-chip memory is less limited and less expensive, accessing off-chip memory is typically slower. Accessing the tables from off-chip memory may be too slow for audio decoding in real time.
Further limitations and disadvantages of conventional and traditional systems will become apparent to one of skill in the art through comparison of such systems with the invention as set forth in the remainder of the present application with reference to the drawings.